Internet-Draft SDF Relations December 2022
Laari Expires 15 June 2023 [Page]
ASDF Working Group
Intended Status:
Standards Track
P. Laari

Extended relation information for Semantic Definition Format (SDF)


The Semantic Definition Format (SDF) base specification defines set of basic information elements that can be used for describing a large share of the existing data models from different ecosystems. While these data models are typically very simple, such as basic sensors definitions, more complex models, and in particular bigger systems, benefit from ability to describe additional information on how different definitions relate to each other. This document specifies an extension to SDF for describing complex relationships and additional information about them.

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This Internet-Draft will expire on 15 June 2023.

1. Introduction

The Semantic Definition Format (SDF) [SDF] is a format for domain experts to use in the creation and maintenance of data and interaction models in the Internet of Things. The SDF specification defines a generic data model that can be used as a meta model when converting between other data models, such as IPSO Smart Objects or Digital Twins Definition Language (DTDL) [DTDL]. SDF model defines a set of affordances, describing the interfaces for the Object. These can be mapped to corresponding affordances in other data models.

The base specification defines ways to represent parent-child relations between two definitions. However, sometimes there is a need to describe also more complex relations to support arbitrary connections between definitions and also referring to definitions outside of the SDF models. These could be, for example, defining possible location of a device inside a room, how a device is controlled by another device, or physical topology between devices. This enables defining more complex systems using SDF models.

The basic parent-child relations between SDF Objects and Things can be defined by including a definition of a child in the definition of the parent. This covers a large share of simple data models defining, e.g., simple sensors, or more complex devices containing a set of sensors. On the other hand, SDF can be used also to describe even more complex entities, such as buildings with rooms and other related objects inside a building. When we extend the SDF usage, the simple parent-child relation is often not enough, but more complex relations may be needed to describe the connections between the definitions. These relations can be for example physical (e.g., an object is inside another object), functional (e.g., an object can control another object), or semantic (e.g., an object is similar to a term defined in another ontology).

This document extends the base SDF specification by adding a new keyword to describe also other relations between physical or logical objects. This new keyword is needed to describe, without loss of information, models from ecosystems that are using complex relation information in their definitions.

This extension enables describing relations from SDF models to various (SDF or other) definitions. For a link data type for affordances, e.g., for a link property that can be accesses and modified during runtime, the "sdfType for links" extension [I-D.bormann-asdf-sdftype-link] can be used instead.

NOTE: This extension is now defined based on the Relationships feature in the DTDL specification. There may be other kind of definitions for relationships in other data models that must be taken into account and this specification may need to be extended to cover also those requirements.

2. Terminology

This specification uses the terminology specified in [SDF], in particular "Class Name Keyword", "Object", and "Affordance".

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. SDF Relation Extension

In this section we define a new SDF Class Name Keyword, sdfRelation, that can be used to describe complex relations. The definitions are on class-level, i.e., the sdfRelation keyword does not give any instance specific information about the relation, but defines the potential relations between definitions.

3.1. Namespaces

The SDF namespace block can be used to provide CURIE prefixes for external ontologies for use with sdfRelation extension. For example, in case of SAREF (Smart Applications REFerence ontology) ontology extension for buildings [saref4bldg], we can use the following namespace definition:

  "namespace": {
    "saref": ""

3.2. Qualities of sdfRelation

In this section, the qualities of the sdfRelation are defined. These qualities are used to define the potential type of the connection between the definitions and to which definition the connection can be made.

Table 1
Quality Type Required Description
relType string/IRI? no What kind of relationship these definitions have
target string no Target definition for the relation
description string no Description of the relationship
maxItems integer no Maximum number of instances of the target types
minItems integer no Minimum number of instances of the target types
property object no Additional properties for this relation
writable boolean no Is the target writable or not

3.2.1. relType

The relType quality describes what kind of relationship this definition has to the target definition. This can use different ontologies, such as SAREF from ETSI. The used ontology MUST be defined in the namespace block to give a short name for the ontology IRI.

For example the "relType" field could define the relationship to be saref:isControlledByDevice, when the SAREF ontology is used with CURIE prefix "saref" defined in the namespace block for the full IRI The defined purpose for the relation is a functional relationship between the two definitions.

3.2.2. target

The "target" field defines to which definition or ontology term this definition with sdfRelation has a relation to. This can be e.g. #/sdfObject/room, when the target object is defined in the same SDF model. This may also be left undefined, and in that case the relation may be any other object (Note: This is from DTDL (check), does it make sense in SDF context?)

The target does not have to be another SDF object, but it can be also a reference to another ontology. For example, we may have a Temperature sensor, which relation to SAREF temperature sensor is defined and it is the same as this one.

  "namespace": {
    "exont": "",
    "saref": ""
  sdfObject: {
    "temperature": {
      "description": "Example temperature object",
      "sdfProperty": {
      "sdfRelation": {
        "sameAs": {
          "relType": "exont:same-as",
          "target": "saref:TemperatureSensor"

3.2.3. description

The description of the relationship. For SDF version 1.1, the description is a string. (For future SDF versions this description can be localizable, allowing different languages in the description.)

3.2.4. maxItems

Maximum number of instances of the target definition that can be related to this definition. If not specified, the number of instances is not limited.

3.2.5. minItems

The minimum number of instances of the target definition that must exist for this definition. If defined, this value MUST be between zero and maxItems. Default: 0.

3.2.6. property

Object with key-value pairs that describe additional properties for this relationship. Details TBD.

3.2.7. writable

Is the information of the relation writable, i.e., can be changed. Default: false.

3.3. Example relation description

In the following example, we have a definition for first-object which located next to second-object:

  "namespace": {
    "exont": ""
  sdfObject: {
    "first-object": {
      "description": "Example object",
      "sdfProperty": {
      "sdfRelation": {
        "next": {
          "relType": "exont:next-to",
          "target": "#/sdfObject/second-object"
    "second-object": {
      "description": "Example object, next to the first object",
      "sdfProperty": {
      "sdfRelation": {
        "next": {
          "relType": "exont:next-to",
          "target": "#/sdfObject/first-object"

4. SDF DTDL mapping

This section (to be removed) shows mapping between SDF and DTDL qualities for relations.

Table 2
Quality (SDF) Quality (DTDL) Description Required
sdfRelation @type In DTDL, this is "Relationship", this is the objects sdfRelation entity yes
"name-of-relation" name In SDF, this is the entity name yes
relType @id DTDL: The ID of the relationship description no
writable writable Boolean, is this relation writable or not no
target target An Interface ID, in SDF the target definition no
$comment comment This is for model authors in DTDL no
description description DTDL: localizable description for display no
  displayName DTDL: localizable name for display no
property properties A set of Properties that define relationship-specific state no
maxItems maxMultiplicity max nof target instances no
minItems minMultiplicity min nof target instances no

6. IANA Considerations

This document has no IANA actions.

7. References

7.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Koster, M. and C. Bormann, "Semantic Definition Format (SDF) for Data and Interactions of Things", Work in Progress, Internet-Draft, draft-ietf-asdf-sdf-12, , <>.

7.2. Informative References

"Digital Twins Definition Language (DTDL) v2", , <>.
Bormann, C., "An sdfType for Links", Work in Progress, Internet-Draft, draft-bormann-asdf-sdftype-link-00, , <>.
Poveda-Villalón, M. and R. Garcia-Castro, "SAREF extension for building", , <>.


The author wants to thank Ari Keranen, Mikko Saarisalo, and Christer Holmberg for their feedback and comments.

Author's Address

Petri Laari